This invention relates to the field of semiconductor technology, and in particular a method for selectively creating semiconductor nano- and micro-patterns, such as etch patterns or porous patterns.
Considerable interest has been shown recently in porous semiconductors because of their good properties for use in sensors and electroluminescent devices. Silicon is currently the technologically most important semiconductor material. However, applications in semiconductor photonics seemed unlikely due to its indirect electronic band gap. The discovery of electrochemically formed visible light emitting porous Si has, in the past few years, stimulated intense and increasing research activity. The main reason for this tremendous interest is the prospect of light emitting devices made of porous Si.
Prior approaches to producing porous Si patterns mostly use photolithographic techniques. The fabrication of porous semiconductors, for example, of silicon or GaAs, requires a lithography/microfabrication process to create nano- or micro- structures on semiconductor wafers. Examples of papers which describe such structures and techniques for their fabrication: xe2x80x9cDoping-induced selective area photoluminesence in porous siliconxe2x80x9d, A. J. Steckl, J. Xu, H. C. Mogul and S. Mogren, Applied Physics Letters, 62 (16) 1982 (1993); xe2x80x9cFabrication of visibly photoluminescent Si microstructures by focused ion beam implantation and wet etchingxe2x80x9d, J. Xu and A. J. Steckl Applied Physics Letters 65, 2081 (1994); xe2x80x9cEnhancement and suppression of the formation of porous siliconxe2x80x9d, S. P. Dattagupta, C. Peng, P. M. Fauchet, S. K. Kurinec, and T.N. Blanton, Journal of Vacuum Science and Technology B 13, 1230 (1995); xe2x80x9cIon-irradiation control of photoluminescence from porous siliconxe2x80x9d, J. C. Barbour, D. Dimos, T. R. Guilinger, M. J. Kelly, and S. S. Tsao, Applied Physics Letters 59, 2088 (1991); and xe2x80x9cSilicon-based visible light emitting devices integrated into microelectronic circuitsxe2x80x9d, K. D. Hirschmann, L. Tsybeskov, S. P. Dattagupta, and P. M. Fauchet, Nature (London) 384, 338 (1996).
Large area porous surfaces on semiconductor substrates are typically produced in two. ways, both of which use a wet etch process. One method uses an electrochemical process in which a small electric current is passed through an electrolyte to the substrate to etch the sample. A second method uses a stain etch to accomplish the same end with no externally applied electric current. The entire exposed area of the substrate is etched. The creation of nano- or micropatterned porous semiconductors requires some technique for restricting the etch process to areas within the nano- or micro- pattern and inhibiting the process elsewhere. Several methods have been proposed to restrict the etch process to the micropattemed areas.
In one technique, an electrochemical resistant mask, which allows the electrolyte to reach the semiconductor surface only in areas where a porous surface is required, is bonded onto the substrate. The disadvantage of this method is that one must apply an electrochemical resistant layer (such as Si3N4) to the substrate, pattern it using photolithography, etch the pattern through the electrochemical resistant layer without etching the substrate, electrochemically etch to produce the porous surface, and then remove the electrochemical resistant layer. This is an unduly complex process. The smallest structures that can be created are limited by the resolution of the photolithography/microfabrication of the chemical resistant layer.
In another technique, the substrate is amorphized (i.e. the crystal structure is destroyed) by implanting ions at a high dose in areas where a porous surface is not desired. Amorphizing the sample requires a large dose of ions which affect the electrical characteristics of the material. Since the amorphization is required everywhere where a porous surface is not desired, and since the other areas may be used for electrical finctions, this process can leave the semiconductor in a state unsuitable for use in a device. The implanted ions are impurities in the sample. In addition, after amorphization, a high temperature anneal is required to regrow the crystal structure. Since impurities in the semiconductor migrate during this heating process, the total anneal time is limited for the completed device (thermal budget). This one step alone will use a substantial part of this limited anneal time.
Photo-assisted electrochemistry can be employed wherein the sample is illuminated through an optical mask while the surface is etched electrochemically. The resolution of the photo-assisted electrochemical process has both a photolithography (diffraction) limit and a limit imposed by the lifetime of the optical carriers as they migrate from the illuminated region into the dark region. These optical carriers are necessary for etching, so the etched boundary will be diffuse (or smeared).
The surface can be ion milled (dry etched) in areas where a porous surface is required. The electrochemical etching process is enhanced in areas which have been ion milled. The ion milled surface etches faster than the non-ion milled surface. So the surface is etched everywhere. This process is not totally selective. Another drawback is that the ion milling produces a shallow trench.
Alternatively, advantage can be taken of the fact that the incubation time for pore formation is strongly dependent on the impurity concentration (doping) of the substrate. By locally doping the wafer and then the annealing substrate, micropattemed porous silicon has been produced by stain etching under illumination. If the incubation time of the neighboring areas exceeds the total etch time in the implanted areas, a totally selective etch process is achieved. If there is no annealing, the process is a partially selective negative process similar to the amorphization process described above.
This latter process is totally selective only if the incubation time contrast between implanted regions and the non-implanted regions is greater than the total etch time required in the implanted regions. If not, a porous surface is produced everywhere, with significantly enhanced etching in the implanted regions. Dopant ions are used which modify the electrical characteristics of the substrate (but only in the neighborhood of the porous Si areas). A thermal treatment is required using some of the thermal budget.
An object of the invention is to alleviate the disadvantages of the prior art.
According to the present invention there is provided a method of selectively creating a patterned semiconductor by electrochemical etching, comprising the steps of treating selected regions of a semiconductor to reduce the threshold potential at which pore formation occurs during an electrochemical etch; and carrying out an electrochemical etch on said semiconductor at an anodic potential at least equal to said reduced threshold potential for said selected regions and less than the threshold potential for untreated regions to selectively form patterns in said selected regions.
The formed patterns are typically patterns of porous material, when it is desired to form a porous semiconductor, but can also be highly defined etch patterns, for example, lines, gratings, or pits, formed in the implanted regions.
The invention depends on the novel effect discovered by the inventors that the threshold potential at which a porous surface is created can be selectively reduced where the near (sub) surface of a semiconductor is modified, preferably by implanting ions, although other means could be employed). The electrochemical step is performed in the range between the. reduced threshold potential and the normal threshold anodic potential required to produce pores on the untreated substrate. This is attributed to Schottky barrier breakdown, which is facilitated due to an inhomogeneous field distribution at a defect site.
The choice of etchant is determined by the substrate material. Typically, the etchant will be hydrochloric acid, hydrobromic acid, or hydrofluoric acid.
Typically the substrate is silicon and the implanted ions are silicon ions so that no impurities are introduced into the substrate by the ion implantation process. The etchant in this case may be hydrofluoric acid. Other types of ions can be used.
Alternatively, the substrate may be GaAs, in which case the etchant may typically be hydrochloric or hydrobromic acid. Desirably, in the case of a doped semiconductor, the implanted ions should be the same as the dopant. Typically, in the case of GaAs, the dopant and implanted ions would be Si.
In order to form pores, the substrate needs to be partially dissolved. In the case of an electrochemical etch of silicon with HF, the Si has to be oxidized at or near the wafer surface. This happens when holes (h+) are present at or near the wafer surface. For ntype material, at positive potentials the material is depleted in holes and thus does not dissolve. However, light with an energy greater than or equal to the band gap will create electron-hole pairs. The holes migrate to the wafer surface and can cause dissolution to occur in untreated areas. The process in accordance with the invention, and in particular the electrochemical etch, should therefore normally be carried out in under xe2x80x9cdarkxe2x80x9d conditions, by which it is understood that no significant amount of light is present having an energy greater than or equal to the band gap to thereby avoid the presence of photoinduced carriers. It will be understood by one skilled in the art that illumination that does not create photoinduced carriers will of course have no significant effect on the process and the term xe2x80x9cdark conditionsxe2x80x9d as used in this specification is considered to include the presence of such illumination that does not create significant photoinduced carriers in the semiconductor.
The sensitization of the substrate can be achieved by other means. In case of both GaAs and silicon, a scratch on the surface can be used for sensitization of the substrate. Other techniques for creating crystal defects can be used to initiate the process according to the invention.
Thus, in accordance with the invention the threshold for pore production is modified, for example, by implanting ions into the substrate. The process is totally selective, which means the substrate is not modified in any way except in the porous areas. In the case of ion implantation, the implant doses are less than those required for amorphization.
The smallest structure that miay be created is limited only by the ion beam dimensions or, in the case of a broad beam implantation, by the mask used to limit the exposure to those areas which are to be made porous. The process is simple, requiring only an implantation and an electrochemical step.